This invention relates to microwave coupler technology, and more particularly to a high directivity, low insertion loss, ultra-compact coupler and method of manufacturing the same.
Couplers are typically used in applications such as GSM/CDMA, WLAN 802.11a/b/g, and WiMax 802.16d/e to monitor the output power level of a power amplifier (PA) module. Minimizing coupler insertions loss is critical for maximizing PA efficiency especially for battery powered hand held devices. Improved coupler directivity is required to more accurately provide closed loop power control feedback to the base-band when the hand held device is subjected to mismatch conditions.
Conventional CDMA/GSM and WLAN modules use discrete band-limited thin film ceramic couplers in radio chipsets which have high insertion loss and consume substantial board space. Also, conventional WLAN RF power amplifier modules use on-chip resistive and/or capacitive coupling. This approach results in a large variation detector voltage error due to voltage standing wave ratio (VSWR) mismatch.
In other known coupler designs with microstrip transmission lines, the transmission lines have an inhomogeneous dielectric which is partly dielectric substrate and partly air. This inhomogeneous medium results in unequal odd and even mode phase velocities. The difference in the odd and even mode phase velocities causes poor coupler directivity when the coupled length is less than a quarter wavelength.
Several techniques for improving coupler directivity have been proposed. In one approach, the gap between coupled lines is serrated to slow down the odd mode phase velocity without affecting the even mode phase velocity. In another approach, lumped capacitors/inductors are added at each end of the coupler to make even and odd mode phase velocity equal at a particular frequency and improve isolation and directivity. In yet another approach, multiple dielectric permittivities and thicknesses are chosen in a multi-layer substrate stack-up to achieve improved directivity with overlapping quarter wavelength transmission lines. While these and other known techniques may improve upon various performance parameters, no technique has yet been disclosed which can yield a broadband coupler with high directivity, low insertion loss, and small footprint that can be monolithically integrated in a RF integrated circuit.
Thus, there is a need for a broadband monolithic coupler with high directivity, low insertion loss and a compact layout, and a method of manufacturing the same.